This invention relates to lasers and more particularly to a multiple wavelength laser.
In certain high gain molecular lasers such as a transversely excited-atmospheric (TEA) CO.sub.2 laser, simultaneous oscillations at a plurality of different wavelengths can occur. For example, a CO.sub.2 laser may have simultaneously oscillating transitions at more than 10 different wavelengths between 9 and 11 microns. For certain applications, it is desirable to selectively operate this laser at any one of these wavelengths and to have the capability of instantly switching the operation from one wavelength to the other. An example of an application requiring this capability is a laser radar system.
Existing techniques for tuning laser wavelengths involve the insertion and/or adjustment of optical elements within the laser resonator. These techniques include the following:
1. Prisms. Dispersive prisms are utilized to separate the components of the potential laser operating spectrum in angle. By adjusting the laser cavity axis or prism alignment appropriately, only the desired wavelength is aligned for optical resonance to produce a useful output.
2. Diffraction gratings. Spectrally dispersive gratings are used in the same manner as with the prisms described above.
3. Absorbing filters. Materials which exhibit wavelength dependent absorption effects are used to provide spectral tuning or selection. When such a material is placed within a laser resonator capable of oscillating over a particular spectral region, the filter in general perturbs the oscillation so as to produce an output in the spectral regions of simultaneous low-loss/high-gain. Tuning is effected by varying the optical density or spectral characteristics of the absorption spectrum of the filter.
4. Etalons. Interference effects between one or more parallel intracavity mirrors produce a spectrally varying transmission function. When such a device is placed within a laser resonator capable of oscillating over a particular spectral region, the etalon is general perturbs the oscillation so as to produce an output in the spectral region of simultaneous low-loss/high-gain. Tuning is effected by mechanically tilting, varying mirror spacing, or varying the refractive index of intra-mirror media (e.g., by changing gas pressure).
5. Birefringent Tuners. By using a combination of birefringent elements and polarizers, a device is constructed which exhibits a spectrally dependent transmission function and which operates within a laser resonator in the manner similar to that described for etalons. Tuning is effected by either mechanically rotating or temperature tuning of the birefringent elements.
6. Dielectric coatings. In a manner analogous to etalons, multilayer dielectric coatings exhibiting wavelength dependent transmission or reflection functions are used to implement spectral control. Typical tuning techniques involve change in coating design and mechanical tilt.
Difficulties of the above techniques for spectral tuning or selection are summarized as follows:
A. Mechanical instability (1, 2 and 4).
B. Insufficient selectively for use with high gain lasers (1, 3, 4).
C. Excessive losses (2, 3, 5).
D. Excessive complexity (1, 5, 6).
In addition to these areas of generic weakness, a particular problem exists which restricts the use of wavelength tuners or selectors in many applications, that is, the inability of conventional tuning techniques to rapidly and reliably change wavelengths under long term use in fieldable systems. To understand the problem fully, the case of a moderate pulse rate laser radar system using spectral differential absorption or scattering may be considered. Typical requirements involve the use of a CO.sub.2 pulsed laser producing outputs at 10 wavelengths from 9 to 11 microns. Each successive pulse must be at a different wavelength, and 100-200 Hz pulse rates are required. This performance implies that the spectral tuner must provide fractional spectral shifts, .DELTA..lambda./.lambda., of 2% within a period of 5-10 milliseconds. While all of the above techniques may be capable of achieving the spectral range, none is believed capable of achieving the desired speed in an operating laser. This is seen in the case of elements requiring mechanical rotation of resonator alignment sensitive components, that is, it is in general impossible to accelerate, position, stop and stabilize a finite sized optical element within the required times. Such systems are unreliable and produce laser outputs which suffer fatal flaws in the areas of pulse energy instability, excessive beam divergence, and unacceptable boresight jitter.
Other approaches, using continuous rotation of optical elements have proven unworkable for similar reasons. Etalon approaches, using pressure tuning or mechanical (piezoelectric) cavity length tuning, are somewhat more attractive from the point of view of stability, but require the use of extremely thin (and hence delicate) cavity spacings to achieve the required free spectral range and also do not appear compatible with rapid tuning.
In addition to the above prior art techniques, a multiline laser resonator with a double grating wavelength selector is described in an article entitled "An Independently Controllable Multiline Laser Resonator and Its Use In Multifrequency Injection Locking", by A. Javan, et al, Applied Physics Letters, Vol. 29, 588 (1976). Wavelength selection is accomplished by means of apertures adjacent to the plane HR mirror. The disadvantage of this technique is that each aperture has approximately the same diameter as the beam, i.e., approximately 1 cm, and therefore the shutters which are moved relative to the apertures to select various wavelengths have the same size. The mass and inertia of such shutters make this technique impracticable for applications requiring switching speeds of less than 10 milliseconds. Moreover, spectral selectivity, i.e., the capability of separating closely spaced transition lines, is limited by this technique because operation is confined to the near field where spatial separation of dispersed sub-beams is minimal.
This invention is directed to a laser system having improved wavelength selection capability.